Conventionally, it has been widely recognized in golf players that, as a good condition for a ball driving action within a speed range of 20 to 70 m/sec, a golf ball had better be provided with circular dimples on the surface in comparison with no dimples because the dimples make the driving drag smaller and also make the driving distance longer than that with no dimples if the same ball initial velocity is given. Further, it has been also recognized that a bigger circular dimple is preferred at a lower driving velocity for a golf ball while a smaller circular dimple is preferred at a higher driving velocity for a golf ball.
Therefore, recently in order to increase the lift-drag ratio (lift/drag) which affects the driving distance, a lot of research has been done for testing a dimple pattern, a peripheral form of dimple and a cross-section of dimple. For example, there were proposed a concept of triangular dimples and their dimple pattern (PTL 1), a concept of section ally cone-shaped dimples provided with triangular periphery (PTL 2) and a concept of polygonal dimples provided with ridges between dimples (PTL 3).
In other words, it has been well known to the golf players that the dimples are necessary to increase the lift-drag ratio and some land areas on the golf ball surface and some ridges between dimples are also necessary to increase the lift-drag ratio.
On the other hand, in order to get the official approval according to R&A rules it is requested that the balls should follow the standards concerning the ball weight, the ball size, the initial velocity, the symmetric property and so on. Among the R &A standards, especially the reflection coefficient is strictly circumscribed to a specific range because it has a huge effect on the initial velocity and then the driving distance. Therefore, multi-piece balls such as three or four-piece balls have been proposed and some new materials have been improved in order to obtain a longer driving distance within the standard range of the reflection coefficient according to R&A rules.